This invention relates generally to active vibration control (AVC) systems used for attenuating undesirable vibrations, and more particularly, to an AVC system utilized for attenuating vibrations that are generated by an internal combustion engine and transferred to a vehicle body.
Internal combustion engines generate vibrations having frequency components that are related to the rotational speed of the engine. These engine vibrations are transferred to a vehicle body through engine mounting units used to anchor the engine to the vehicle body. The transferred vibrations can be quite annoying to vehicle passengers, and it is generally desirable to reduce such vibrations to improve passenger comfort.
In the past, active vibration control systems have been used to reduce engine generated vibrations that are coupled to the vehicle body. This is typically accomplished by inversely vibrating the vehicle body by generating canceling vibrations having frequency components with essentially the same amplitudes as the transferred engine vibrations, but shifted by 180 degrees in phase. The canceling vibrations combine with the transferred engine vibrations to reduce overall vibration of the vehicle body.
Because the amplitude and frequency content of engine generated vibrations can vary quite significantly with changes in engine rotational speed, digital signal processing and adaptive filtering techniques are generally used when implementing vehicle AVC systems. One such system is disclosed in U.S. Pat. No. 5,146,505, which has issued to Pfaff et al. and has been assigned to the same assignee as the present application. In this system, a signal having frequency components corresponding to those of the engine generated vibrations is derived from a sensed engine rotational speed signal. This derived signal is then applied to the input of an adaptive filter and is transformed by the filter characteristics into an output signal. The adaptive filter output signal is used for driving a cancellation actuator, such as an electromechanical vibrator mounted on the vehicle body. The vibrator produces canceling vibrations that are superimposed with the undesirable engine generated vibrations that are transferred to the vehicle body. An error sensor, such as an accelerometer is used to measure the combined vibration of the vehicle body, and to develop a representative error signal, which provides information to the AVC system for modifying the characteristics of the adaptive filter to minimize the overall vibration level of the vehicle body.
Such systems adequately attenuate engine generated vibrations when the vehicles are stationary. However, when the vehicles are moving over a road surface, it has been found that these AVC systems can exhibit excessive use of power and instabilities, which prevent the efficient and effective cancellation of engine vibrations that are transferred to the vehicle body.
Consequently, there exists a need for an active vibration control system that is efficient and effective in reducing engine generated vibrations that are transferred to a vehicle body not only when the vehicle is stationary, but also when the vehicle is driven over a road surface.